High-fat diet-induced obesity triggers alveolar bone loss and spontaneous periodontal disease in growing mice

Significant increases in body weight and serum cholesterol levels, together with significant
decreases in bone quantity and quality, were found in HFD-induced obese mice. In the
trabecular bone of these mice, deterioration of the trabecular bone architecture resulted
in an overall decrease in mandibular BV/TV, as determined by micro-CT. Although cortical
bone formation was slower in HFD-fed than in control mice, bone formation on the periosteal
surface increased with age in both groups. Additionally, bone resorption on the endosteal
surface was slightly higher in HFD-fed mice than controls, as seen on micro-CT. The
HFD-fed mice at 19 weeks also had a significantly lower in Ct.BD than that of the
controls, consistent with a significant increase in the porosity of cortical bone
at the end of the experiment.

This study demonstrated a significant increase in serum leptin levels in HFD-fed mice
compared with their age-matched controls, although the levels also increased in the
latter.

Leptin is known as an important circulating signal that inhibits food intake and enhances
energy expenditure through its actions in the brain 16]. However, several studies have shown that a HFD plays key role in the development
of leptin resistance in animals 17], 18]. In another study, energy expenditures were lower in mice fed a high-fat versus a
low-fat diet, even though intake was similar between two groups 19]. Energy expenditure inhibition due to leptin resistance leads to abnormal accumulation
of triglycerides in the liver and other organs, since ingested triglycerides are not
being used as an energy source.

We found that serum triglyceride levels tended to be lower and serum HDL cholesterol
levels higher in HFD-fed mice than in control mice. These findings are in contrast
to those of previous studies 20], 21] but are consistent with those of Graham et al. 22]. The differences may be related to whether serum leptin levels exceed the capacity
of the body’s transport system, including the entry of leptin into the cerebrospinal
fluid. Additionally, in HFD-fed mice, an increase in serum total cholesterol levels
is probably followed by an increase in serum HDL cholesterol levels.

The causes of leptin resistance are unclear, but hyper-nutrition leads to endoplasmic
reticulum stress, and thus to inflammation, in adipose tissue 23]–25]. Oxidative stress was also shown to induce leptin resistance in HFD-fed mice 26]. The inflammation in HFD-fed mice is accompanied by increased expression of inflammatory
cytokines, such as interleukin (IL)-6, IL-1?, and tumor necrosis factor-?, in adipocytes
and macrophages via activation of the c-Jun N-terminal kinase and nuclear factor-?B
pathways 24], 26]. Recently, Dib et al. reported that leptin acts as a pro-inflammatory adipocytokine
in peripheral tissues 27]. These studies suggested that endoplasmic reticulum stress induces inflammation by
mediating leptin signals in the adipose tissue of HFD-fed mice.

Other studies have shown a selective increase in the production of reactive oxygen
species (ROS) in the adipose tissues of obese mice. These ROS cause both oxidative
and endoplasmic reticulum stress, including inflammatory changes in adipose tissue
during the course of adipocyte hypertrophy 28]. Moreover, ROS and oxidative stress inhibit osteoblastogenesis 29], 30], suggesting that ROS also inhibit periosteal cortical bone formation in growing HFD-fed
growing mice.

The increased expression of inflammatory cytokines and leptin elicit osteoclast activity
by regulating the RANKL/RANK/OPG pathway, resulting in increased bone resorption 31], 32].

Thus, in this study, the deterioration of bone structure in the HFD-fed mice could
have been due to multiple forms of oxidative stress, which induced leptin resistance
and increased inflammation in adipose tissue. However, an age-dependent increase in
serum leptin levels within the physiological range did not affect gradual bone growth.

We also showed that HFD-induced obesity during growth increases the risk of mandibular
bone osteoporosis and spontaneous periodontal disease. A recent report proposed a
link between systemic osteoporosis and periodontal bone loss based on significant
up-regulation of inflammatory cytokines in bone and in the bone marrow cells of rats
with osteoporosis 33].

In this study, HFD-induced alveolar bone loss may have reflected a state of non-invasive
and non-infective inflammation, such as that characteristic of autoimmune disorders.
This is in contrast to previous studies that used models of experimental periodontitis
8], 9], as periodontal disease develops as a result of the continuous interaction between
host cells and subgingival pathogenic bacteria 34].

The HFD-induced alveolar bone loss in our mice may have been triggered by bacterial
endotoxin [lipopolysaccharide (LPS)], a potential inflammatory mediator in mice with
HFD-induced obesity 35]. In a recent study, a HFD increased alveolar bone loss in mice injected with LPS
36]. In general, bacterial endotoxins are present in large quantities in the gut 37], and clinical studies have reported the development of postprandial endotoxemia following
a high-fat meal 4], 38]. These findings suggest that dietary fats promote the translocation of bacterial
endotoxins from the gut into the circulation, where they stimulate periodontal inflammation
and alveolar bone loss.

Recently, Suganami et al. proposed the concept of “homeostatic inflammation” in the
pathogenesis of non-infectious inflammatory diseases 39]. This may account for the HFD-induced alveolar bone loss in our mice, in which systemic
inflammatory changes in bone and other tissues may have developed in association with
metabolic stress.

In a previous study, rats fed a high-cholesterol diet showed a modest increase in
the distance between the cement-enamel junctions and the alveolar bone crest. The
authors suggested that osteoclastic function plays a major role in alveolar bone resorption
during increased oxidative stress 40]. However, at a gross histological level, there was no evidence of alveolar bone crest
resorption in any of the groups, despite significant increases in serum total cholesterol
levels in the HFD-fed mice. These results likely reflected cortical bone formation
on the periosteal surface during the growth period. By contrast, the PDL fibres in
HFD-fed mice were disrupted, with loss of orientation with respect to the bone surface,
and the normal narrowing of the PDL space was inhibited. Narrowing of the periodontium
narrows with age, as seen in the control mice, is accompanied by increasing acellular
cementum formation and alveolar bone formation. The increased vascular permeability
due to inflammatory changes in the blood vessels of the periodontium may promote monocyte
adhesion to endothelial cells and migration. In addition, osteoclasts differentiated
from those monocytes may have then attached to the alveolar bone surface, resulting
in increased alveolar bone resorption in the HFD-fed mice.

Together, these findings suggest that the spontaneous deterioration of periodontal
bone is a consequence of HFD-induced obesity during growth.

Two limitations to our study must be noted. First, in the histological evaluation,
HFD-fed mice had clear alveolar bone resorption and inflammatory structural changes
in the PDL, but systemic bone metabolism was not assessed using serum analyses, such
as those measuring bone resorption markers and inflammatory cytokines. Second, bone
resorption due to insulin resistance was not considered. Further studies are needed
to elucidate the mechanisms of inflammatory alveolar bone resorption induced by a
HFD.